The present invention relates to hydrostatic drive systems, and more particularly, to such drive systems of the “dual path” type, having a pair of drive wheels (or tracks, etc.), each of which is driven by a separate, separately-adjustable hydrostatic transmission (HST).
A number of different types of vehicles use some form of dual path HST propel arrangement, and it is not intended that the present invention be limited to any particular type or configuration of vehicle. Therefore, and as an example, although the present invention will be illustrated and described in connection with a pair of drive wheels, it should be understood that the invention could be utilized also on a tracked vehicle, and references hereinafter to “driven wheels” will be understood to mean and include any of the known, possible types of outputs.
An example of a vehicle of the type which could utilize the present invention is a lawn care or turf care vehicle (a mower) especially one of the “wheel-steered” type, i.e., a vehicle in which steering is accomplished by driving one of the driven wheels at a different speed than the other driven wheel. By way of further example, the present invention may be utilized on vehicles of the “ZTR” type (zero-turn-radius) in which either driven wheel may be driven in either the forward or reverse direction, independently of the other driven wheel.
In the conventional dual HST propel system, the left and right driven wheels are driven by left and right hydrostatic transmissions, each HST including a motor having its output shaft adapted to drive its respective driven wheel. Typically, the motor is fixed displacement, but the pump hydraulically coupled to the motor is of the variable displacement type. It should be understood that the pump and motor of the HST may be of the axial piston type, or of the radial piston type, or of any other type wherein the pump displacement may be varied by variations in a manual input to the pump, such as a rotatable control shaft. By use of the term “manual” input, it should be understood that the invention could be utilized in a system having pumps with direct, mechanical operator input, or in one having pumps where the displacement is controlled by some sort of hydraulic servo system of the type now well known in the art.
The present invention relates especially to such dual HST propelled vehicles of the type in which the input from the operator, to select the vehicle speed, is transmitted to a speed control crank arm, to rotate the arm about its axis in an amount corresponding to the desired speed of the vehicle. Attached to the speed control crank arm are identical linkage arrangements, one to control the displacement of the pump on the left HST and the other to control the displacement of the pump on the right HST. As is well known to those skilled in the art, the two linkage arrangements are designed and intended to be identical so that, as the operator selects a desired vehicle speed, and rotates the speed control crank arm, the same input motion will be transmitted to the displacement control mechanism of both of the left and right pumps, for the purpose of transmitting the same drive speed to each of the driven wheels.
However, as is also known all too well by those skilled in the art, it is one thing to design the two propel paths (and the control linkages) to be identical, but it is another matter entirely to actually produce and install a pair of HST propel systems on a vehicle, and have them provide exactly the same speed of rotation of the driven wheels for any given operator input to the speed control crank arm. For example, there may be slight, unintended, differences between the linkage arrangements for the left and right HST (or slight differences in the displacements of the left and right HST pumps or motors). Finally, in some cases, there may be a slight difference in tire size which can result in a noticeable deviation from the intended “straight-tracking”.